Future lightwave systems are expected to accommodate large numbers of transmission channels separated by small guard bands. The transmission channels operating at high data rates are planned utilize more fully the existing available bandwidth of single mode optical fibers for delivery of network and other services such as entertainment television. As system planners continue to make trade-offs between design parameters such as coherent and non-coherent approaches, direct and heterodyne detection techniques and the like, it is increasingly apparent that frequency modulation of a single frequency light source such as a distributed feedback (DFB) laser has become an attractive approach for the lightwave transmitter design.
Frequency modulation is often preferred over amplitude or intensity modulation at high data bit rates because chirping and current switching problems, both of which arise from current variations on the light source, decrease the desirability of amplitude and intensity modulation systems. For intensity modulation, large amounts of current must be switched rapidly to the light source. The amount of drive current is typically in the range of 30-60 mA for semiconductor lasers. As the current to the semiconductor laser varies, it causes a small but significant amount of frequency modulation in the laser called chirp. Chirping causes a broadening of the spectral linewidth of emitted radiation. Obviously, such spectral spreading penalizes even the best single frequency light sources. Semiconductor lasers especially DFB lasers have been improved through better fabrication techniques to have a lower linewidth enhancement factor and, thereby, a reduced susceptibility to chirping. Even with such improved light sources, lightwave systems employing amplitude and intensity modulation may have substantial drawbacks when compared with frequency modulation lightwave systems.
The appeal of frequency modulation for lightwave communication systems can be related to the fact that it permits a simplified transmitter design. By directly modulating or varying the injection current to a semiconductor laser, it is possible to modulate the frequency of the laser. For single frequency semiconductor lasers, the carrier density effect which shows a change of frequency with injection current, .DELTA.f/.DELTA.i, is sufficiently large, generally, several hundred Mhz/mA, to minimize residual intensity modulation effects for the frequency excursions required by most FM systems. However, nonuniformity exists for the FM response of such lasers over the modulation bandwidth because of competition between temperature and carrier density effects on the laser frequency.
Nonuniform FM response is viewed with respect to thermal cutoff of the single frequency laser. Below the thermal cutoff frequency, the FM response is extremely large in magnitude on the order of Ghz/mA whereas it is opposite in phase to the FM response above the thermal cutoff frequency. Far above the thermal cutoff frequency, the FM response approaches several hundred Mhz/mA while gradually reversing phase with respect to that below the thermal cutoff frequency. As a result, lower frequency components of a modulated optical signal undergo severe waveform distortion due primarily to temperature or thermal modulation effects on the active region of the laser.
Frequency modulation based lightwave communication systems using lasers whose FM response is nonuniform suffer degradation. In an M-ary FSK system, nonuniform FM response causes drift of a transmitted frequency representing one of M levels per symbol whenever the laser remains at that frequency for a time which is significant as compared to a thermal time constant for the laser. As the frequency drifts, crosstalk increases resulting in degraded bit error rate performance and, ultimately, causes complete failure of the affected link for the lightwave system.
These problems can be ameliorated to some degree by limiting the length of non-alternating data patterns to effectively eliminate the low frequency components of the data sequence. There are other approaches commonly employed for working with the nonuniform FM response of the laser which employ a modulation format or data encoding scheme to also avoid the low frequency modulation region. In one example, Manchester coding is employed with its concomitant penalty of increased system bandwidth requirements. Additionally, problems such as power consumption and device complexity preclude the use of most encoding and modulation techniques. Active and passive equalization networks have been combined with DFB lasers to overcome distortion induced by the nonuniform FM response of the DFB laser. In theory, these networks compensate the nonuniform FM response of the DFB laser by using combined pre-distortion, post-distortion and feedback control methods to realize a somewhat uniform FM response. Both active and passive equalization techniques generally result in relatively small FM response and, therefore, increased drive current requirements. While the combination appears to have a uniform FM response, it is important to realize that the DFB laser itself exhibits a totally nonuniform FM response.
Phase-tunable DFB lasers have also been proposed to overcome the nonuniform FM response problem. These devices are generally fabricated to include two distinct regions: a DFB region for operating as a standard DFB laser and a modulation region without a grating separately contacted for modulating the DFB laser signal. In this way, carrier density effects are artificially controlled through electrode partitioning to achieve quasi-uniform FM response and chirp suppression. Quasi-uniform FM response for two-electrode DFB lasers is reported up to several hundred megahertz. However, DFB regions employed in these devices exhibit unwanted nonuniform FM response and are primarily designed to have inherently low linewidth enhancement factors for chirp suppression. Moreover, design and fabrication complexity together with operational speed limitations caused by the multi-section structure diminish its desirability for use in future lightwave systems.
While the alternatives described above have been proposed and demonstrated for dealing with the nonuniform FM response of directly modulated lasers, in particular, DFB lasers, it has been noted recently that "[t]he potentially most rewarding solution is to construct a laser having an inherently uniform FM response." J. of Lightwave Tech., Vol. 7, No. 1, pp. 11-23 (January 1989). As noted in the descriptions above, each laser element still exhibits an inherent nonuniform FM response. Upon realizing this fact, the authors of the above-cited article lament as follows, "[u]nfortunately, the goal of obtaining single mode operation, high output power, narrow linewidth, long life, along with a uniform FM response, in a wide selection of commercial devices at various wavelengths, is still elusive."